Abstract
In this investigation, the use of phosphotungstic acid (PWA) and phosphomolybdic acid (PMA) as well as Zn2+ containing kaolin and bentonite explored for chemical recycling of post-consumer poly(ethyleneterephthalate) (PET) wastes have been explored. The clay supported catalysts containing 5wt% of the metals and heteropolyacids (HPAs) synthesized using wet impregnation method. Nitrogen adsorption and desorption studies, SEM–EDX mapping, powder XRD, FTIR and XPS analysis have evaluated effect of metal ions and HPAs loading on the surface area, pore volume, elemental composition and crystalline nature. Total surface area of BET increased with a loading of 5 wt% of Zn2+, PWA and PMA on kaolin and bentonite, while the pore volume and pore diameter remain unchanged. SEM and EDAX mapping images showed that the heteropolyacids crystals are well dispersed on the surface and occupied interlayer spaces of the clay support. SEM–EDX showed that bentonite showed a better loading of PWA and PMA compared to kaolin. PET waste water bottles collected from the local market used for the chemical recycling process. The aminolysis reaction using Zn2+ and PWA loaded on bentonite showed complete depolymerisation of PET wastes to produce 87–98% of BHETA. The glycolysis reaction using the above catalysts showed complete depolymerisation at 180–210 °C and yielded 78–90% of BHET. When comparing the clay, bentonite performed well in terms of heteropolyacid loading and afforded a higher yield of BHET and BHETA because of higher loading of Zn and HPA, as supported by SEM–EDX and XPS. We also examined reusability of the catalysts for glycolysis.
Highlights
The chemical recycling process for post-consumer polyesters such as PET and PLA could contribute to the development of circular economy, in which the valorisation of polyesters through chemical depolymerization into value added monomers (Thomas et al 2020; Payne and Jones, 2021)
The effect of metal ions and HPAs loading on the surface area, pore volume, elemental composition and crystalline nature of the kaolin and bentonite has been evaluated by nitrogen adsorption and desorption studies, SEM-EDX mapping, powder XRD, FTIR and X-ray photoelectron spectroscopy (XPS) analysis
We have investigated depolymerisation of the post-consumer PET soft drink bottles collected from the local market using glycolysis and aminolysis process catalysed by heteropolyacids impregnated kaolin and bentonite catalysts and the same compared with Al3+ and Zn2+ loaded clay analogues
Summary
The chemical recycling process for post-consumer polyesters such as PET and PLA could contribute to the development of circular economy, in which the valorisation of polyesters through chemical depolymerization into value added monomers (Thomas et al 2020; Payne and Jones, 2021). In an eco-friendly approach, ash obtained from orange peel (Lalhmangaihzuala et al, 2021) and bamboo leaf fly ash catalyst (Laldinpuii et al, 2021) were used as a catalyst for PET depolymerization that yields 79 and 83% of the recrystallized product, respectively These catalysts are non-toxic, renewable, accessible and inexpensive catalysts for glycolysis of post-consumer polyethylene terephthalate waste. To prepare economically viable catalysts, commercially available clay supports were purchased and the ion was exchanged with Al3+, Zn2+ and phosphomolybdic and phosphotungstic acids and characterized using BET, XRD, FT-IR, XPS and SEM and the structures of depolymerized products, N1,N4 -bis(2hydroxyethyl)terephthalamide (BHETA) and bis(2-hydroxyethyl)terephthalate (BHET) is confirmed by 1H NMR, 13C NMR and mass spectra. The catalytic activity of 5wt% of Zn2+, Al3+, PWA and PMA loaded kaolin and Bentonite has been studied toward glycolysis and aminolysis of PET waste using ethylene glycol as glycolyte reagent and ethanolamine as aminolyte reagent under normal atmospheric conditions except reaction temperature (Scheme 1). The mass spectrum recorded using (Shimadzu, LCMS-2020, ionization method: EI) and recorded the FT-IR spectrum using the KBr pellet technique (Shimadzu, IRTracer 100)
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